This invention relates to the field of optical storage systems and, more specifically, to a detection module used in optical storage systems.
Magneto-optical (MO) storage systems provide storage of data on rotating disks. The disks are coated with a magneto-optical material and divided into magnetic areas referred to as domains. The data is stored in the magneto-optical material as spatial variations in the magnetic domains.
In one type of MO storage system, a MO head is located on an actuator arm that moves the head along a radial direction of the disk. As the disk rotates, the head can be positioned over a particular domain. A magnetic coil on the head creates a magnetic field oriented perpendicular (i.e., vertical) to the disk""s surface. Each vertically magnetized domain represents either a zero or one depending on the direction that the magnetic field is pointing.
The vertical magnetization is recorded in the MO material by focusing a beam of laser light to form an optical spot on a disk domain. The laser beam heats the MO material at the spot to a temperature near or above the Curie point. The Curie point is the temperature at which the magnetization in the material may be readily altered with an applied magnetic field. A current is then passed through the magnetic coil to orient the vertical magnetization in either the up or down direction indicating a one or zero. This orientation occurs only in the region of the optical spot where the temperature is sufficiently high, and remains after the laser beam is removed.
Information is read from a particular domain using a less powerful laser beam, making use of the Kerr effect, to detect a rotation of polarization of a beam reflected off the disk""s surface. The magnetization of each domain causes a rotation of the optical polarization of the laser beam incident at the domain. The polarization of the reflected beam is rotated in a direction, clockwise or counter-clockwise, determined by the orientation of the domain""s vertical magnetization. Measurement of the direction of rotation is performed by an optical detection system that converts the optical signals into electrical signals.
One particular MO storage system uses a radio frequency modulated Fabry-Perot laser source coupled to a single optical fiber to transmit the laser beam to the storage system""s head. The optical fiber directs an incident laser beam to the head, which is then directed toward the rotating disk. The head also directs the reflected laser beam from the rotating disk to the same optical fiber. Discrete optical components are located remotely from the head for optically processing the rotated polarization components of the reflected laser beam. This system relies on the preservation of the polarization states of the reflected beam through the entire optical path. As such, a polarization maintaining optical fiber is used in the system. However, the birefringent nature of a polarization maintaining fiber combined with certain characteristics of the laser diode causes some undesirable results.
Birefringence is a characteristic of an optical material in which the index of refraction depends on the direction of polarization of light propagating in the material. Birefringence in the fiber material causes a phase shift between the orthogonally polarized light beams that are transmitted along the fiber. In addition, the use of a radio frequency modulated Fabry-Perot laser diode produces a relatively broad-spectrum, multi-wavelength incident light beam having wavelengths that fluctuate with time.
One problem with using a single optical fiber system is that noise, associated with the FP laser, is transmitted by the optical fiber to the discrete optical processing components located remotely from the head. Because of the birefringent characteristic of the optical fiber, the multiple fluctuating wavelengths of the incident light from the laser diode cause signal components in the reflected beam to have polarization states that also fluctuate. The competition between the multiple fluctuating wavelengths of the reflected beam appears as noise at the storage system""s detectors. This noise may limit the achievable data rate at a given signal level. Furthermore, optical fibers exhibit polarization mode leakage that may cause one polarization mode to appear as another. This polarization leakage also appears as noise at the storage system""s detectors.
Another problem with the single fiber optical system is that the use of optical processing components located remotely from the head require tight alignment tolerances between the discrete components. Such tight alignment tolerances cause the manufacturing of the system to be more difficult.
FIG. 1 illustrates another MO storage system 100 that uses multiple optical fibers and discrete optical components on a head. A single optical fiber is used to direct incident light and two optical fibers are used in the return path to direct reflected light. In system 100, discrete optical processing components are placed directly on the head 10 in a particular relationship to each other in order to direct the incident light beam onto the disk and to convert the polarization information from a magnetized domain into two separate reflected light beams having differential intensity information. The discrete optical components may include a leaky beam splitter 20, a phase plate 30, and a polarizing beam splitter (or other polarization splitting element such as a Wollaston prism) 40.
Linearly polarized light 50 is directed through leaky beam splitter 20 and then to the disk (not shown) by a mirror 60. The return beam 70 is directed back to leaky beam splitter 20 with a Kerr rotation as discussed above. The linearly polarized light, in system 100, is characterized by two plane-polarized beams: one beam with its electric field parallel to the plane of incidence (horizontal or p-polarized) and the other beam with its electric field perpendicular to the plane of incidence (vertical or s-polarized). The leaky beam splitter 20 reflects the return light beam 75 to phase plate 30. Phase plate 30 introduces a phase shift between the p-polarized and s-polarized components of light 75. The phase plate 30 rotates the polarization such that equal components of p-polarized and s-polarized light are received by polarizing beam splitter 40 when a Kerr rotation is not present on the disk. When a Kerr rotation is present on the disk, however, phase plate 30 rotates the polarization such that different components of p-polarized and s-polarized light are received by polarizing beam splitter 40. The light passing through the phase plate 30 is directed to the polarizing beam splitter 40.
The polarizing beam splitter 40 includes a glass plate having a multi-layer coating on its front surface that acts to separate the s-polarized and p-polarized components into spatially separate beams. The coating on the front surface of polarizing beam splitter 40 reflects light based on its polarization such that all of the s-polarized light 80 is reflected and all of the p-polarized light 85 is transmitted. The s-polarized light 80 is reflected to a lens 90 that focuses it into a multimode fiber which carries the light to a detector (not shown). The p-polarized light 85 is refracted to the back surface of polarizing beam splitter 40. The back surface of polarizing beam splitter 40 acts as a mirror to reflect the p-polarized light 85 such that it is offset from the s-polarized light 80. The p-polarized light 85 is transmitted to a second lens 95 that focuses it into a second multimode fiber which carries the light to a different detector. The detectors convert the light amplitude signals from each channel into electrical signals.
Once the electrical signals are produced, a difference signal between the s-polarized and p-polarized light is calculated. The difference signal is used to determine the sign of the Kerr rotation indicating the direction of the magnetic domain. When equal s-polarized and p-polarized components are received, the detectors would generate identical signals and, thus, the differential signal would be zero. If unequal components of light are received, the detectors would generate a negative or positive signal depending on the Kerr rotation direction of the reflected light.
The MO storage system""s head has various channels and recesses that are dimensioned and positioned to hold the discrete optical components in a particular relationship to each other in order to direct the light beams as described above. This system reduces sensitivity to laser mode competition and polarization leakage by analyzing the polarization state of the reflected beam on the head instead of returning the reflected beam through the input polarization maintaining fiber for analysis.
One problem with such a system, however, is the need to separately fabricate, cut, and align each of the discrete optical components mounted on the head. This may significantly increase the cost and assembly time of the head. Another problem with such a system is the larger size of the head due to the space required for each of the discrete optical components and the return path optical fibers placed on the head. The size of a head limits the spacing that can be achieved between MO disks in a storage system. The size and mass of the head also limits tracking bandwidth, track density, and the speed at which data can be accessed from the MO disk.
The present invention pertains to an optics module having a birefringent crystal. A first plurality of segmented optical coatings may be integrated with the birefringent crystal to optically process a first light through a path in the birefringent crystal. The first plurality of segmented optical coatings may be integrated with the birefringent crystal based on the path.
Additional features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.